Crystal contacts of which one element is silicon



Patented Apr. 6, 1948 CRYSTAL CONTACTS OF WHICH ONE ELEMENT IS SILICONCharles Eric Ransley, Sudbury, England, assignor to The General ElectricCompany Limited,

London, England No Drawing. Application February 24, 1944, Se-

rial No. 523,748. In Great Britain March 22,

16 Claims.

This invention relates to electrical crystal contact devices of the typecomprising a semi-conducting crystal of which silicon is a, mainconstituent and a metallic contact member in contact with it, thecontact member commonly taking the form of a fine wire having one endpointed and biased into contact with the crystal; the invention relatesalso to methods of preparing the crystal of such contact devices.

As will presently be explained in greater detail, in the United Statespatent application Serial No. 454,290, filed August 10, 1942, now PatentNumber 2,419,561 dated April 29, 1947, it was proposed to introduce intothe silicon, freed from most or all of the impurities usually present incommercial silicon, a small proportion of aluminum or beryllium, topolish a fragment of the resulting materi'al, to treat its surface withhydrofluoric acid,

and then to subject it to a controlled oxidation. The main advantagesaccruing from this process are that the resulting contact devices arerendered more uniform and have greater ability to withstand withoutdetriment larger values of power at the operating frequency.

One of the uses of contact devices of. the type specified is as mixersof electric oscillations of very high frequency, for example 3000 mc./s.Their efficiency for this purpose at this high frequency, and even moreat still higher frequencies, increases as the capacity of the contactdecreases. The object of this invention is to provide crystals thatyield contact devices of smaller capacity than those prepared accordingto the methods particularly described in the said application, orrathersince crystals resulting from apparently the same process oftenyield contact devices differing widely in capacity-to provide a methodof making the crystals that produces a larger proportion of crystalsyielding contact devices having very small capacity.

This can be achieved by introducing into the silicon a considerableproportion of chromium or molybdenum, or of a mixture of chromium andmolybdenum. Chromium is generally preferable to molybdenum, because itusually enables the contact device to Withstand larger values ofwavesignal power than does molybdenum. Preferably the addition ofchromium or molybdenum, or a mixture thereof, is introduced inaccordance with the invention into the silicon, which is first freedfrom its usual impurities, at the same time that a small quantity ofberyllium or aluminium or both isintroduced into the silicon. Theproportion of chromium or molybdenum to silicon is one not much less,and possibly greater, than 10% by weight.

One embodiment of the invention will now be described by way of example.In this example the additive characteristic of the invention ischromium. Little or no other change would be required if it weremolybdenum or a mixture of chromium and molybdenum.

Commercial silicon is purified, as described in the aforementionedapplication, by subjecting the silicon to the process described by N. P.Tucker in the Journal of the Iron and Steel Institute, vol. CXV (1),page 412, (1927). This process removes only those impurities that arenot contained in the silicon crystals but occur as an additional phase,usually in the interstices between the silicon crystals. Commercialsilicon is usually prepared by cooling the melt slowly; then practicallyall of the deleterious impurities are in the interstices and can beremoved by the Tucker process. If that process fails with any sample ofsilicon,

it is probably because the silicon has been quenched rapidly; then thesilicon should be melted and cooled slowly before it is subjected to theTucker process. After the commercial silicon is thus purified, theadditive metals are preferably introduced by melting a mixture of thepure silicon and the additive. During this process, care must be takennot to introduce undesired impurities. Those that might be derived fromthe air (a small quantity of oxygen is not necessarily deleterious) canbe avoided by melting in. a suitable atmosphere; the most suitable wehave found is a vacuum, that is to say, residual gas at a pressure ofnot more than 0.001 mm. of mercury. But it is difiicult or perhapsimpossible to find any crucible to contain a melt that does not reactslightly with the melted silicon. This diiiiculty can be overcome byusing a crucible consisting of or lined with pure beryllium; for thenthe only impurity introduced from the crucible is beryllium, which isnot harmful but beneficial. Thus, the purified commercial silicon ismelted in vacuo in a beryllia crucible with 20% of its weight ofchromium and with enough beryllium and alu minium to give a content of4% beryllium and aluminium in the final product. Some of the berylliummay be derived from the crucible as previously mentioned. After the melthas been slowly cooled it is treated in the manner described in theUnited States patent application No. 468,577, filed December 10, 1942,now Patent No. 2,428,992 dated October 14, 1947; namely, before thesolid product comprising the cooled melt is divided into pieces eachsuitable for a single crystal, the product is maintained for aconsiderable time at a temperature greatly above atmospherictemperature, but below the melting point of the product, and thenquenched rapidly. The time and temperature of this heat treatment ischosen such that the additive is distributed approximately uniformly inthe quenched product. theoretical grounds, it is probable that theminimum temperature at which this heat treatment will produce thedesired result is that at which the additive is completely soluble inthe silicon. Since the solubility of the additive is a maximum at theeutectic temperature, the minimum temperature mentioned cannot be higherthan the eutectic temperature which is about 575 C. for aluminium and1090 C. for beryllium. The eutectic temperature of thealuminium-berylliumsilicon appears not to be known. It has been foundthat when the additive is one-quarter per cent. of aluminium, heating at575 C. to 580 C. for several hours will produce the desired result. Butif the amount of the additive is less than that corresponding to theeutectic, which will certainly be the case when the additive isberyllium, the minimum temperature may be much below the eutectictemperature; moreover there is no reason Why temperatures approachingthe minimum should be used. Accordingly, these theoreticalconsiderations are of little value as a guide to practice. Whether theadditive is aluminium or beryllium or a mixture of the two (We havefound one-quarter per cent. aluminium and one-half per cent. berylliumvery suitable), maintaining the product at 1050 C. for one hour and thenquenching in water has been found to produce the desired uniformity. Afragment of the product is then subjected to the steps (3), (4), (5),(6), (7), of the aforementioned Patent Number 2,419,561. In the step (3)a fragment of the cooled melt has a plane part of its surface highlypolished by any of the processes customary among metallurgists, forexample grinding with emery of increasing fineness and finally polishingwith alumina-or magnesia or both. In the further step (4) the wholesurface of the fragment is then dipped for 5 minutes into commerciallypure 40% hydrogen fluoride diluted with an equal quantity of water. Themain purpose of this treatment with hydrofluoric acid is to prepare thepolished surface for the subsequent oxidation; accordingly it is notnecessary, though it is usually convenient, that the unpohshed part ofthe surface should be dipped.

In the further step (5) the necessary surface oxidation is performed.body resulting from step (4) is placed on a fiat silica tray (togetherpossibly with other bodies at the same stage of preparation) andintroduced into a silica tube furnace filled with air at a regionthereof maintained at 1050" C. There is a definite optimum temperaturefor this treatment; the temperature should be observed, e. g. by athermocouple, and kept as near 1050 C. as possible. The body ismaintained in the said region for 2 hours and then allowed to cool. Anatmosphere of oxygen at a controlled pressure may be substituted for theair in the furnace; then the time of heating will depend on the pressureof oxygen.

In the further step (6) the treatment given in step (4) is repeated. Nowpart of its object is to prepare the unpolished part for step (7)accordingly the unpolished part must be dipped. In the process now beingdescribed in detail the dipping of the polished part is equallynecessary, in

For this purpose the 4 order that part of the oxide layer formed in step(5) should be removed. After the dipping, the body is washed with water.

In the further step (7) a metallic layer is deposited on some or all ofthe unpolished surfaces. The layer must not be deposited on the polishedsurface; accordingly this is first protected .by

coating the flat end of a rod with a layer of adhesive, which may be thematerial known as monostyrene, and pressing the polished surface againstthis layer. The exposed unpolished surface is then coated with copper bya method described by Bedel in Comptes Rendus, vol. 192 (1931), page802. It consists merely in dipping the surface for 5 to 10 seconds intoa solution of cuprous oxide in 20% hydrofluoric acid; the copper layerdeposited may be subsequently thickened by electrolysis. Copper is notdeposited in tungsten or molybdenum by this process; accordingly thesaid rod may be conveniently of tungsten or molybdenum. The coatedsurface is then washed and dried. The body can then be mounted in asuitable metal capsule by means of solder or some other suitablelow-melting alloy intervening between the metal layer and the cap sule.The rod is then detached from the body and the polished surface of thebody cleaned from the adhesive by washing with a suitable solvent, suchas benzene or ethyl acetate.

I claim:

1. In the manufacture of an electrical crystal contact device of thekind in which the semi-conducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the step of introducing into the silicon a determinatequantity of at least one of the two metals chromium and molybdenum, thequantity of such additive being at least of the order of 10% of thesilicon.

2. In the manufacture of an electrical crystal contact device of thekind in which the semi-conducting crystal contact element is mainly,silicon, the production of said silicon contact element by a processwhich includes the step of introducing into silicon that issubstantially freed from the metallic impurities common in commercialsilicon a determinate quantity of at least one of the two metalschromium and molybdenum, the quantity of such additive being at least ofthe order of 10% of the silicon, and a determinate relatively smallquantity of at least one of the two metals aluminium and beryllium,whichform a solid solution with the silicon.

3. In the manufacture of an electrical crystal contact device of thekind in which the semiconducting crystal contact element is mainlysilicon, the production of'said silicon contact element by a processwhich includes the step of introducing into silicon that issubstantially freed from the metallic impurities common in commercialsilicon a determinate quantity of at -least one of the two metalschromium and molybdenum, the quantity of such additive being at least ofthe order of 10% of the silicon, and a determinate quantity of at leastone of the two metals aluminium and beryllium, which form a solidsolution with the silicon, the quantity of such latter additive bein ofthe order of between /2% and 1% of the silicon. r

.4. In the manufacture of an electrical crystal contact device of thekind in which the semiconducting. crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of purifying commercial silicon so as toeliminate therefrom substantially all its common metallic impurities,and thereafter melting the purified silicon in vacuo in a crucible atleast the interior surfaces of the walls of which are composed of pureberyllia, with between and of its weight of at least one of the twometals chromium and molybdenum and with enough of at least one of thetwo metals aluminium and beryllium to give a content of substantially/2% of the latter additive in the final product;

5. In the manufacture of an electrical crystal contact device oi' thekind in which the semiconducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of purifying commercial silicon so as toeliminate therefrom substantially all its common metallic impurities,and thereafter melting the purified silicon in vacuo in a crucible, atleast the interior surfaces of the walls of which are composed of pureberyllia, with substantially 20% of chromium and with enough aluminiumand beryllium to give a content of substantially 4% of each of said lasttwo metals in the final prodnot.

6. In the manufacture of an electrical crystal contact device of thekind in which the semiconducting crystal contact element is mainlysilicon, the production of said silicon contact elernent by a processwhich includes the steps of introducing into silicon that issubstantially freed from the metallic impurities common in commercialsilicon a determinate quantity of at least one of the two metalschromium and molybdenum, the quantity of such additive being at least ofthe order of 10% of the silicon, and a determinate quantity of at leastone of the two metals aluminium and beryllium, which form a solidsolution with the silicon, the quantity of such latter additive being ofthe order of between /2% and 1% of the silicon, polishing a plane partof the surface of a fragment of the resulting product, thereaftertreating saidfragment with hydrofluoric acid, thereafter heating saidfragment in an oxidising atmosphere, thereafter treating said fragmentwith hydrofluoric acid so as to remove some but not all of the oxidelayer formed during said heating, and finally depositing a metalliclayer on at least part of the unpolished surface of said fragment,

'7. An electrical crystal contact device including a semi-conductingcrystal contact element comprised of silicon which contains a quantity,of the order of at least 10% of the silicon, of at least one of the twometals chromium and molybdenum, said element having a polished andoxidised surface, and a metallic contact element cooperating with saidsurface of said crystal element.

8. An electrical crystal contact device including a semi-conductingcrystal contact element comprised of silicon which contains a quantity,of the order of at least 10% of the silicon, of at least one of the twometals chromium and molybdenum, together with a determinate relativelysmall quantity of at least one of the two metals aluminium andberyllium, said element having a polished and oxidised surface, and ametallic contact element cooperating with said surface of said crystalelement.

9. An electrical crystal contact device including a semi-conductingcrystal contact element comprised of silicon which is substantially freefrom the metallic impurities common in commercial silicon but whichcontains a quantity,- between substantially 10% and 20% of the weight ofsilicon, of at least one of the two metals chromium and molybdenum,together with a quantity, between substantially and 1% cf the weight ofsilicon, of at least one of the two metals aluminium and beryllium, saidelement having a polished and oxidised, surface, and a metallic contactelement co-operating with said surface of said crystal element.

10. An electrical crystal contact device inc1uding a semi-conductingcrystal contact element comprised of silicon which is substantially freefrom the metallic impurities common in commercial silicon but whichcontains substantially 20% of chromium, of aluminium and of beryllium,these percentages being of the weight of silicon, said element having apartially etched-away oxidised and polished surface, and a metalliccontact element co-operating with said surface of said crystal.

11. An electrical crystal contact element comprising silicon and aquantity, of the order of at least ten per cent by weight of thesilicon, of at least one of the 'tWo metals chromium and molybdenum.

12. .An electrical crystal contact element comprising silicon, aquantity of the order of at least ten per cent by weight of the siliconof at least one of the two metals chromium and molybdenum, and adeterminate relatively small quantity of at least one of the two metalsaluminium and beryllium.

13. An electrical crystal contact element comprising silicon, a quantitybetween substantially ten per cent and twenty per cent by weight of thesilicon of at least one of the two metals chromium and molybdenum, and aquantity between substantially one-half per cent and one per cent byweight of the silicon of at least one of the two metals aluminium andberyllium.

14. An electrical crystal contact element comprising silicon which issubstantially free from the metallic impurities common in commercialsilicon, twenty per cent of chromium by weight of the silicon, andone-quarter per cent of aluminium and one-quarter per cent of berylliumby weight of the silicon.

15. An electrical crystal contact element comprising silicon and aquantity of the order of at least ten per cent by weight of the siliconof at least one of the two metals chromium and molybdenum, said elementhaving at least one polished and oxidised surface area.

16. An electrical crystal contact device comprising silicon, betweensubstantially ten per cent and twenty per cent by weight of the siliconof at least one of the two metals chromium and molybdenum, and betweensubstantially one-half per cent and one per cent by weight of thesilicon of at least one of the two metals aluminium and beryllium, saidelement having at least one polished and oxidised surface area. i

CHARLES ERIC RANSLEY.

file of this patent:

UNITED STATES PATENTS Number Name Date Feussner et al. Feb. 5, 1935

